Translation of abstract (English)

The molecular motor protein, myosin, converts chemical energy from ATP hydrolysis into useful mechanical work that is used to translocate the myosin filament along an actin filament during muscle contraction. The mechanism involved in the chemo-mechanical coupling necessary for myosin function is poorly understood. In this research work, a computational attempt is being made to understand the recovery-stroke mechanism in a myosin molecule which is one of the fundamental processes that occurs during muscle contraction in living organisms. During the recovery stroke, the myosin motor is primed for the next power stroke bya 60 degree rotation of its converter domain (which bears the lever arm). This reversible motion is coupled to the activation of its ATPase function through conformational changes along the relay helix, which runs from the Switch-2 loop near the ATP to the converter domain. This coupling mechanism is determined by computing minimum energy pathways (MEP) between the crystallographic end-states of the recovery stroke, yielding a continuous series of optimized intermediates in atomic detail. The MEP reveals a two-phase mechanism, in which the successive formation of two hydrogen bonds by the Switch-2 loop is correlated with the successive movement of the two helices that hold the converter domain: the relay helix and the SH1-helix. The first phase involves the formation of a hydrogen bond (between Gly457, on the N-terminal of the relay helix and the (gamma-phosphate of ATP) which causes a �See-Saw� like motion of the relay helix. The second phase is triggered by the formation of another hydrogen bond (between Switch-2 and the Ser181 of the P-loop) which causes the wedging of a loop against the N-terminal end of the SH1-helix, resulting in the longitudinal translation of the SH1-helix relative to the relay helix. The converter domain first responds to the �See-Saw� motion of the relay helix by rotating, 20 degrees, then to the translation of the SH1-helix by rotating a further 40 degrees. The proposed coupling mechanism is consistent with the existing mutational data and explains the role of a highly conserved loop structure, called here as the �Wedge loop�, which was recognized for the first-time. Molecular dynamics simulations of Dictyostelium discoideum myosin II in the two end conformations of the recovery stroke with different nucleotide states (ATP, ADP.Pi, ADP) reveal that the side-chain of Asn475 (which initiates the first-phase of the recovery-stroke) switches away from Switch-2 upon ATP hydrolysis to make a hydrogen bond with Tyr573 (on the Wedge loop). This sensing of the nucleotide state is achieved by a small displacement of the cleaved gamma-phosphate towards Gly457 which in turn pushes Asn475 away. The sensing plays a dual role by (i) preventing the wasteful reversal of the recovery stroke while the nucleotide is in the ADP.Pi state, and (ii) decoupling the relay helix from Switch-2, thus allowing the power stroke to start upon initial binding to actin while Gly457 of Switch-2 keeps interacting with the Pi (known to be released only later after tight actin binding). The catalytically important salt bridge between Arg238 (on Switch-1) and Glu459 (on Switch-2), which covers the hydrolysis site, is seen to form rapidly when ATP is added to the pre-recovery stroke conformer and remains stable after the recovery stroke, indicating that it has a role in shaping the ATP binding site by induced fit.